The present invention relates to a mass spectrometer and a method of mass spectrometry. A preferred embodiment of the present invention relates to a mass filter arranged upstream of an analytical quadrupole rod set mass filter or mass analyser.
Quadrupole rod set mass filters are well known and comprise four rod electrodes. A RF voltage and a resolving DC voltage are simultaneously applied to the rod electrodes so that the mass filter operates in a mass or mass to charge ratio resolving mode of operation. As will be well understood by those skilled in the art, when a quadrupole rod set mass filter is operated in a mass resolving mode of operation ions having mass to charge ratios within a desired mass to charge ratio range will be onwardly transmitted by the mass filter but undesired ions having mass to charge ratios outside of the mass to charge ratio range will be substantially attenuated. Ions which are not desired to be onwardly transmitted by the mass filter are attenuated either by causing the ions to assume unstable trajectories such that the ions impact upon the rod electrodes or else by radially ejecting the ions from the mass filter.
One problem with conventional quadrupole rod set mass filters operated in a mass resolving mode of operation is that they suffer significantly reduced performance at increased pressures. This is in part due to the axial cooling of the ions which results in prolonged transit times and a corresponding increased exposure to the resolving RF and DC electric fields. In particular, if ions which are desired to be onwardly transmitted by the mass filter are present for too long within the mass filter and experience too many cycles of the applied RF voltage then the ions can be inadvertently radially ejected. As a result, the ion transmission efficiency of the mass filter for ions which are desired to be onwardly transmitted by the mass filter will be significantly reduced.
Another significant problem with conventional arrangements is that analytical quadrupoles can be exposed to relatively high ion currents with the result that a relatively large number of undesired ions possess unstable trajectories through the analytical quadrupole with the result that a relatively large number of ions are not onwardly transmitted by the analytical quadrupole. A relatively high percentage of the ions which are not onwardly transmitted impinge upon the rods of the analytical quadrupole. As a result the rods of the analytical quadrupole can quickly become contaminated and the analytical quadrupole will suffer form a loss of resolution due to surface charging effects.
As will be understood by those skilled in the art, cleaning the rods of an analytical quadrupole rod set is a non-trivial task which requires a service engineer and for the mass spectrometer to be taken offline.
U.S. Pat. No. 6,987,264 (Whitehouse) discloses an arrangement wherein at least two multipole ion guides are arranged adjacent to each other. U.S. Pat. No. 6,987,264 (Whitehouse) is not concerned with the problem of the rods of an analytical quadrupole becoming contaminated.
It is desired to provide an improved spectrometer and method of mass spectrometry.